Problems with heated front windscreen

I've got a 06 plate Focus Zetec Climate which has developed a problem with the heated front windscreen.

When I switch it on to clear the screen before I drive I hear a knocking noise coming from the drivers side of the engine compartment. This does not seem to happen when the engine is warmer however I don't often have the need to use it then!

The only difference that the heated screen will make is the huge draw of power when its switched on. Cant really account for the knocking, could be alternator. You dont have the blowers on full demist too do you??? If you do that will be using the air conditioning too, could be air conditioning pump if you do.

The only difference that the heated screen will make is the huge draw of power when its switched on. Cant really account for the knocking, could be alternator. You dont have the blowers on full demist too do you??? If you do that will be using the air conditioning too, could be air conditioning pump if you do.

Thanks for this.

I bang the whole lot on when i get in, didn't know that i wasn't supposed to!

I will try it without the blowers and see if it still happens.

I don't fancy getting a new alternator/air con pump though, i was hoping that it would be an easy, cheap and well known fix.